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Yamaha B-2 - Unstable bias and very tight adjustment range

Thanks for your hints rottalpha. There's no short on the pcb. What's Q10 ?
The bias trimmer is a side screw type for ease of adjustments. Since it has a different footprint, and since only two pins are used, I soldered the cursor to the unused pin. This allowed soldering the trimmer by its outer pins, which fit in the space of the former trimpot pins used.
 
david winter said:
Thanks for your hints rottalpha. There's no short on the pcb. What's Q10 ?
The bias trimmer is a side screw type for ease of adjustments. Since it has a different footprint, and since only two pins are used, I soldered the cursor to the unused pin. This allowed soldering the trimmer by its outer pins, which fit in the space of the former trimpot pins used.
Click to expand...
Transistor 10, as in TR 110 in the schematic, IIRC
Try getting a multi-turn trimmers if you can.
 
Oh you meant the one with (10) marked on the pcb. Never mind :) Here's a photo. I took care of not inserting it as the three others.
Of course the legs aren't shorting.

20260603_102639.jpg
 
david winter said:
Oh you meant the one with (10) marked on the pcb. Never mind :) Here's a photo. I took care of not inserting it as the three others.
Of course the legs aren't shorting.

View attachment 3775134
Click to expand...
well, there is one of your problems. The pin-out of Q/TR 10 is wrong. Should be mounted same as the other three in the row.
Yes, I know you looked at the service manual and mounted according to that, which is wrong.
No, you did not look at the other B-2 threads where this was discussed.

Count your blessing that your v-fets are still Okay ;)
 
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:eek: Wow, thanks for this ! I of course didn't notice that the pcb layout was wrong and showed R128 going to the collector instead of the emitter.
Strange that they didn't publish this info in their bulletins.

Edit: TR110 ok on both divers :) DC adjusts fine and protection now clicks. Will check voltages, then put new micas and silicon thermal paste before adjusting the bias.
I just wish I knew about the pcb layout error, this cost me quite some time until you pointed it. Maybe it will be a good thing to update the PDF in the AK Database, and warn about this.
 
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david winter said:
:eek: Wow, thanks for this ! I of course didn't notice that the pcb layout was wrong and showed R128 going to the collector instead of the emitter.
Strange that they didn't publish this info in their bulletins.

Edit: TR110 ok on both divers :) DC adjusts fine and protection now clicks. Will check voltages, then put new micas and silicon thermal paste before adjusting the bias.
I just wish I knew about the pcb layout error, this cost me quite some time until you pointed it. Maybe it will be a good thing to update the PDF in the AK Database, and warn about this.
Click to expand...
This is why I never depopulate and entire board at once and do one part at a time. Too easy to screw up, as Pete stated you got lucky here :)

Nashou
 
I would never put a cursor to an unconnected solder. What I meant is that if the cursor is connected to the former unused pin, the two remaining pins (the outer ones) are soldered to the two tracks, hence the empty loose solder seen on one of the photos.
 
david winter said:
Maybe it will be a good thing to update the PDF in the AK Database, and warn about this.
Click to expand...
David, you can update all the files in the world but the point is that you still have to remove ALL V-FETs from the amp before any work is done. With them removed, mistakes like these won’t amount to much. I’m glad the V-FETs aren’t damaged. Just thinking about it gives me μ heart attacks. Good luck. :biggrin:
 
david winter said:
I would never put a cursor to an unconnected solder. What I meant is that if the cursor is connected to the former unused pin, the two remaining pins (the outer ones) are soldered to the two tracks, hence the empty loose solder seen on one of the photos.
Click to expand...
There’s more information about b-2s in this forum than anywhere in the world.
The due diligence is required before diving into an amp like B-2. … and when still in doubt, ask ;)

Glad you got off unscathed
 
Sure ! :thumbsup:
But I must make a new ground bridge behind the meter board. The original one broke at a fold at one end. I'll put a stripe of adhesive aluminium foil.
 
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david winter said:
Sure ! :thumbsup:
But I must make a new ground bridge behind the meter board. The original one broke at a fold at one end. I'll put a stripe of adhesive aluminium foil.
Click to expand...
Copper foil is a much better choice for grounding. After installation, you could add a protective coat of sorts to prevent copper oxidation.

Edit: Whatever self-adhesive material you end up using for this, make sure the adhesive is conductive. :biggrin:
 
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